Which of the following is the space-weather term for an extreme geomagnetic storm?
Why NOAA's space weather scales use three letters: R for radio blackouts, S for solar radiation storms, and G for geomagnetic storms, each running from 1 (minor) to 5 (extreme). An extreme geomagnetic storm, the kind that can wipe out HF paths and push aurora far south, is therefore rated G5.
Watch out X5 and M9 are solar flare classifications (X and M are X-ray flux classes), which describe the flare itself rather than the resulting disturbance of Earth's magnetic field. B9 is an even weaker flare class.
G for Geomagnetic, and 5 is the top of the scale: G5 = Giant storm. X and M are flare classes, not storm levels.